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| Issue 12, 2024 |
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Extracts from the Internet | ||
Continuous atom laser1 May 1999The simplest `atom' laser was first constructed at MIT in 1997. Unlike its optical counterpart, the atom laser produces not coherent light but coherent atomic beams, using the Bose-Einstein condensate (BEC) of atoms for the purpose. While until recently only very short pulses have been achievable in this way, a continuous 100-ms beam of Rb atoms is now reported by researchers at the Max Planck Institute for Quantum Optics in Germany. In contrast to the broad front propagation of ordinary laser light, an atomic beam they produced was about a nanometer in diameter - i. e., thousands of times narrower than the optical focus - owing to the minimum noise conditions that a specially designed magnetic trap secured for the BEC formation process. Subject to an alternate electromagnetic field of a special configuration, Rb atoms escaped the trap and then fell under the influence of gravity, the coherence of this falling atomic beam following that of the original BEC atoms. In NIST experiments, an atomic laser has been created in which the atomic beam can go not just downwards but in any direction, the choice being controlled by two optical laser beams applied to a sodium BEC. The NIST atomic beam was not a continuous one, however, but rather a quasi- continuous succession of nearly overlapping pulses. A continuous atom laser may be useful in heavy-current measuring devices and in high-tech nanostructure production processes. Source: Physics News Update, Number 422 Wave packet de Broglie wave1 May 1999In 1994 Jacobson and Yamamoto of Stanford University predicted that the de Broglie wavelength of a wave packet of several photons should be shorter than that of any one of the photons, the decrease in the wavelength being inversely proportional to the number of photons. For example, a 1000 photon wave packet of green light was expected to have an x-ray de Broglie wavelength. A Brazil team lead by S. Padua has for the first time verified this prediction for a two-photon case. In the experiment conducted by the team, coherent photon pairs were produced by splitting a photon in an optically nonlinear crystal, after which the interference of two-photon wave packets as well as the interference of either photon in a pair was individually observed and the wavelengths involved were measured. Consistent with the 1994 prediction, a reduction of the de Broglie wavelength of a wave packet to half that of each individual photon was observed. Source: http://publish.aps.org/FOCUS/ Remote galaxy1 May 1999Researchers using the Hubble Space Telescope have seen the remotest galaxy ever detected, and one probably still at the early stages of its formation. The light that now comes to Earth was emitted by the galaxy when the Universe was only a few million years old, i. e., just 5% its present age. The distance to the galaxy was determined from the spectral redshift due to the expansion of the Universe. While there is a huge amount of ultraviolet light in the galaxy's emission, the large redshift it undergoes turns UV optical. The UV emission is probably due to the fact that light from young hot stars scatters on gas clouds present in the galaxy but not incorporated into star material. Source: http://www.nature.com A gamma ray burst from a galaxy core1 May 1999Hubble observations of the optical source identified with the gamma ray burst GRB 970508 have been repeated to reveal that, since the last observation in August 1998, the source has faded to the point that no emission from it can now be detected. At the same time, the new Hubble images reveal the presence of an elliptical galaxy which was earlier obscured by the bright light from the optical source. Importantly, the optical source is only 70 pc or less from the galaxy's centre and must therefore take part in its core processes. The nature of gamma ray bursts is still a matter of debate. According to earlier optical data, they arise in galaxies with a high rate of star formation which may, in the present case, occur in the galaxy's core. Alternatively, the activity of the core may explain the phenomenon. Source: http://xxx.lanl.gov/abs/astro-ph/9903236 |
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics. Permanent editor is Yu.N. Eroshenko. It is compiled from a multitude of Internet sources. | |
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